This invention relates to fiber reinforced composite materials or structures and more particularly to composite material reinforced by three-dimensional fiber elements randomly distributed in a matrix in a structure or material formed by molding.
Initial efforts to make molded chopped-fiber composites employed short one-dimensional (1-D) fibers randomly aligned in a matrix material to provide reinforcement. Randomly oriented, 1-D, chopped fibers are not isotropic and cannot be relied upon to produce homogeneous structures. The random alignment inevitably results in a situation where few or no fibers are oriented in the direction of the applied stress and result in a weak, low-strength structure. Since the location of these weak spots cannot be determined apriori, the entire structure must be designed using low allowable strengths that will encompass the scatter of material properties. This problem has been partially addressed by using pieces of woven cloth as a 2-D reinforcing material randomly distributed in the composite. Woven cloth reinforcement fibers provide good strength in two of the three dimensions, but the random distribution results in almost the same limitation as 1-D reinforced composites.
In an effort to achieve isotropic strength properties in a fiber-reinforced structure, fiber reinforcement in the third dimension has been accomplished by either weaving three-dimensional fiber reinforcing structures or driving the third dimensional fiber through a series of two dimensional woven structures. All of these structures produced a geometrically repeatable pattern of relative positions of one fiber to another with some type of replicated geometry throughout the structure. While these structures accomplished the purpose, they are very expensive.
The present invention is particularly concerned with producing a composite structure with near-isotropic strength properties by molding a fiber element with three-dimensionally oriented fibers in a matrix material. The fiber elements are randomly oriented, overlapping and interlocking (as opposed to a woven geometric array) to produce the structure at a reasonable cost.